Thermal barrier coatings which are applied to components are known from the field of gas turbines, as described for example in EP 1 029 115 or WO 00/25005.
It is known from DE 195 35 227 A1 to provide a thermal barrier coating in a steam turbine in order to allow the use of materials which have worse mechanical properties but are less expensive for the substrate to which the thermal barrier coating is applied. The thermal barrier coating is applied in the cooler region of a steam inflow region.
GB 1 556 274 discloses a turbine disk having a thermal barrier coating in order to reduce the introduction of heat into the thinner regions of the turbine disk.
U.S. Pat. No. 4,405,284 discloses a two-layer ceramic outer layer for improving the abrasion properties.
U.S. Pat. No. 5,645,399 discloses the local application of a thermal barrier coating in a gas turbine in order to reduce the axial clearances.
Patent specification 723 476 discloses a housing which is of two-part design and has an outer ceramic layer which is thick. The housing parts of the one housing are arranged above one another but not axially next to one another.
Thermal barrier coatings allow components to be used at higher temperatures than the base material alone permits or allow the service life to be extended.
Known base materials allow use temperatures of at most 1000° C.-1100° C., whereas a coating with a thermal barrier coating allows use temperatures of up to 1350° C. in gas turbines.
The temperatures of use of components of a steam turbine are considerably lower than in gas turbines, but the pressure and density of the fluid are higher and the type of fluid is different, which means that in steam turbines different demands are imposed on the materials.
The radial and axial clearances between rotor and stator are essential to the efficiency of a steam turbine. The deformation of the steam turbine housing has a crucial influence on this; its function is, inter alia, to position the guide vanes with respect to the rotor blades secured to the shaft. These housing deformations include thermal elements (caused by the introduction of heat) and visco-plastic elements (caused by component creep and/or relaxation).
For other components of a steam turbine (e.g. valve housings), inadmissible visco-plastic deformations have a disadvantageous influence on their function (e.g. leak tightness of the valve).